General Structural Consequences of Bonding Types

The potential energy V of an ionic crystal can be expressed approximately as the sum of the Coulomb interaction and the Born repulsive potential  

Because of the Coulomb repulsion between like ions, the prime condition controlling the crystal structure adopted by an ionic substance is that like ions cannot come in contact. Since anions are generally larger than cations, the limiting factor is the number of anions that may surround the smaller cation in its coordination shell. The limiting value of cation-anion radius ratio RJRa which the anions would come in contact can be calculated for any coordination polyhedron. Such values are listed in Table 1. Thus ionic substances 

Lower Limiting Values of Radius Ratios for Stability of Ionic Coordination Polyhedra 

Coordination about cation Distribution of anions Minimum RJ a for stability 

9 Corners of right triangular prism and outside rectangular faces thereof all edges equal 0.732 

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